1. Field of the Invention
The present invention relates to a power semiconductor device used for controlling high power, and particularly to a nitride-based field effect transistor (FET) of the lateral type.
2. Description of the Related Art
A circuit, such as a switching power supply or inverter, employs a power semiconductor device, such as a switching device or diode. The power semiconductor device needs to have a high breakdown voltage and low on-resistance. The relationship between the breakdown voltage and on-resistance includes a tradeoff relationship determined by the device material. Progress in technical development so far has allowed the power semiconductor device to have a low on-resistance close to the limit determined by silicon, which is the main device material. Accordingly, it is necessary to change the device material, if a lower on-resistance is required.
In recent years, attention is being given to research on a power semiconductor device using a wide bandgap semiconductor. For example, it has been proposed to use a wide bandgap semiconductor, such as a nitride-based material (e.g., GaN, AlGaN) or a silicon carbide-based material (SiC), as the switching device material, in place of silicon. The use of such a semiconductor can improve the tradeoff relationship determined by the device material, thereby remarkably reducing the on-resistance.
A power semiconductor device of a nitride-based material, such as GaN, can realize an on-resistance lower than a power semiconductor device of Si. In this type, a power device having a HEMT (High Electron Mobility Transistor) structure provides an on-resistance as low as 1/100 of the Si limit or less.
In a lateral device, such as the HEMT structure, the electric field distribution between the gate and drain determines the breakdown voltage. Accordingly, if a high breakdown voltage is obtained along with a short distance between the gate and drain, a lower on-resistance can be realized. In this case, it is necessary to relax electric field concentrations at electrode corners. A field plate structure is known as an effective countermeasure against this. For example, Jpn. Pat. Appln. KOKAI Publication No. 2002-118122, and U.S. Pat. No. 6,483,135 disclose a GaN-based power HEMT having a field plate electrode.
FIGS. 14 and 15 are sectional views each schematically showing a conventional GaN-based power HEMT having a field plate electrode.
Each of the HEMTs shown in FIGS. 14 and 15 includes a channel layer 101 of non-doped GaN, and a barrier layer 102 of n-type AlGaN disposed on the channel layer 101. A source electrode 114 and a drain electrode 115 are disposed separately from each other on the barrier layer 102. A gate electrode 113 is disposed between the source electrode 114 and drain electrode 115 on the barrier layer 102. The barrier layer 102 is covered with an insulating film 116 between the gate electrode 113 and drain electrode 115.
In the case of the HEMT shown in FIG. 14, a field plate electrode 117 is disposed on the insulating film 116 and electrically connected to the gate electrode 113. In the case of the HEMT shown in FIG. 15, a field plate electrode 118 is disposed on the insulating film 116 and electrically connected to the source electrode 114. The field plate electrode 118 is arranged to cover the gate electrode 113 with an insulating film 119 interposed therebetween.